1. Field of the Invention
This invention relates to a photovoltaic device, and more particularly to a photovoltaic device that can be mass-produced with ease and has a superior conversion efficiency.
2. Related Background Art
In recent years, as future energy sources for human beings, it is discussed whether we may entirely rely on oil and coal, which are said to make the environment of the earth warm because of carbon dioxide generated as a result of their use, and on atomic energy, which can not be said to be perfectly free from the risk of the radiation leakage that may be caused by any accidents and also may occur even at the time of normal plant operation. Photovoltaic devices are expected to come into much wider use, because their energy source is the sun and they have a very little influence on the environment of the earth. Under existing circumstances, however, there are some problems that obstruct their real spread. In particular, it is a great problem that they require a high production cost for their conversion efficiency (photoelectric conversion efficiency, i.e., the proportion of the energy that can be converted into electric power to the solar radiation energy incident on the surface of a photovoltaic device).
For example, small-area photovoltaic devices making use of single-crystal semiconductors such as Si and GaAs have achieved a conversion efficiency of 24 to 25%, but require much energy and time for producing single-crystal substrates. They also have structures elaborately designed so that the conversion efficiency can be improved, but the process for their fabrication becomes complicated to bring about an extreme increase in production cost. In the case of polycrystalline silicon that enables relatively easy fabrication, the production cost can be reasonably low, but the conversion efficiency is about 17 to 18% as for those having a small area, and about 14 to 15% as for those having a large area. In the case of amorphous silicon (hereinafter "a-Si") that enables much easier fabrication and can achieve a lower production cost, the conversion efficiency is about 12% even as for those having a small area, and about 10% as for those having a large area. Thus, it has been considered difficult for photovoltaic devices to achieve both the improvement in conversion efficiency and the cost reduction at the same time.
In order to improve conversion efficiency, it is advantageous to utilize as effectively as possible the solar radiation to be made incident. The first factor that determines the utilization rate of solar radiation in photovoltaic devices is light absorption characteristics of the semiconductors to be used. In general, when semiconductors capable of even absorbing light having longer wavelengths, the output voltage tends to lower, and the conversion efficiency is not necessarily improved as expected in some cases.
The second factor that determines the utilization rate of solar radiation in photovoltaic devices is to decrease reflection loss of the light made incident on the surface of a semiconductor. In general, semiconductor layers have a great refractive index, which is about 3.4 for Si, about 3.6 for GaAs and about 3.5 for InP in the visible light region, and the reflection of light on the surface thereof reaches about 30%, which is large enough not to be negligible. To solve this problem, it is attempted to provide a reflection preventive layer on the surface of the semiconductor.
In the simplest instance in which one reflection preventive layer is provided, no reflection is known to occur so long as the following relationship is established when, with respect to light having a wavelength .lambda., refractive index of the exterior is represented by no, refractive index of the semiconductor layer by ns, refractive index of the reflection preventive layer by na, and thickness of the reflection preventive layer by da, as shown in FIG. 1. EQU na=(no.multidot.ns).sup.1/2 ( 1) EQU da=.lambda./4na (2)
In the case when the exterior is air, no=1, and hence, when, e.g., the semiconductor layer is formed of Si, ns=3.4. Therefore, a value of na=1.84 is optimum. In the case of ITO (mainly composed of In.sub.2 O.sub.3 and containing a small amount of SnO.sub.2) or ZnO, n=1.8 to 2.0 in approximation, thus this substantially fulfills the condition (1).
Further in accordance with the condition (2), assume that these materials are formed in a thickness of 68 to 72 nm, they come to reflect substantially no light of .lambda.=550 nm, so that the reflection can be reasonably lowered even in the whole visible light region.
In addition, when ITO or ZnO is deposited under appropriate conditions and optionally doped, conductivity .sigma. can be enhanced up to about 10.sup.-4 .OMEGA..multidot.cm, thus it can function as an electrode. Hence, ITO and ZnO are suitable as transparent electrodes, and are widely used in photovoltaic devices. From the viewpoint of production, these materials can also be deposited by early established processes such as vacuum deposition and sputtering, and the cost therefor can also be low when formed in a thickness of about 70 nm.
The photovoltaic devices used as solar cells, however, are exposed to the weather and used over a long period of time in that state. Hence, their surfaces must be protected. What are usually widely used therefor are glass plates and resin films. In the case of crystal photovoltaic devices, they are usually used in the state that glass sheets or wafers are stuck to the surfaces using a transparent adhesive such as PVA (polyvinyl acetate). In the case of a-Si photovoltaic devices, a-Si is deposited on glass substrates and is so designed that light is incident on the substrate side, whereby their weatherability can be ensured. In the case of photovoltaic devices prepared by deposition on flexible substrates such as stainless steel sheets, their surfaces can be protected with fluorine type resin films having a weatherability, to thereby making the most of flexibiltiy. Such films, glass sheets and so forth stuck to the surfaces of solar cells later are hereinafter called protective layers without regard to materials therefor.
Incidentally, the use of protective layers causes an additional problem. Materials for the protective layers usually have a refractive index of about 1.3 to about 1.6 in the visible light region. In the case of Si, its refractive index is ns=3.4. Assume that the refractive index of a protective layer is no=1.5, referring back to the expression (1), its preferable refractive index na is 2.25. Such a refractive index is unsuitable for ITO or ZnO, and hence there occurs a reflection loss of 5% or more. Meanwhile, ZnS (na=2.3) and TiO.sub.x (na=2.2 to 2.7 depending on production processes) are known as transparent materials having a high refractive index, and these can theoretically solve these problems. These materials, however, have so much a higher resistance than ITO and ZnO that these can not well serve as electrodes. When used inevitably, comb-shaped grid electrodes having a high density must be used in combination, so that the shade thereof virtually makes the utilization rate of incident light lower and the incident light is still not well utilized. From the viewpoint of manufacture, when sputtering is carried out, the resistance of targets must be made higher, and hence a high deposition rate can be achieved with difficulty, bring about a disadvantage.
As methods for improving the utilization rate of incident light, it is well known to form the reflection preventive layer in double layers. In this case, there are various manners of designing. To apply such a method to semiconductors, when refractive index of the transparent electrode on the exterior side (the protective layer side) is represented by na1, its thickness by da1, refractive index of a transparent electrode on the semiconductor layer side by na2, and its thickness by da2, as shown in FIG. 2, the following conditions may be fulfilled. EQU no.multidot.ns=na1.multidot.na2 (3) EQU cos.sup.2 .delta.=(na2.sup.2 .multidot.no-na1.sup.2 .multidot.ns)/(na1+na2).multidot.(no.multidot.na2-na1.multidot.ns)(4)
where EQU .delta.=.pi..multidot.na1.multidot.da1/.lambda.=.pi..multidot.na2.multidot. da2/.lambda. (4')
So long as the foregoing is fulfilled, the reflectance becomes zero at two wavelengths and can be decreased over a wide range, so that the utilization rate of incident light can be made much higher. However, if it is attempted to apply this planning to an instance where light is incident on the protective layer side or transparent electrode side, it follows that, when, for example, the refractive index of Si, ns is 3.4 and the protective layer refractive index no is 1.5 like the previous instance, the expression (3) gives na1.multidot.na2=5.1. Assume that ITO is used in the transparent electrode on the exterior side, a value of na2=2.55 is optimum from the expression (3) (wherein na1=2.0), so that this planning can not be carried out because no suitable material can be found among materials (having a low resistance) usable as the transparent electrode on the semiconductor layer side. Thus, because of restrictions in respect of physical properties of the materials for transparent electrodes, the light incident on photovoltaic devices is not well utilized. Hence, in respect of conversion efficiency, too, there is still room for improvement.